Structure of cementum

Two main types of cementum could be identified on the basis of the presence or absence of cells.

1. Acellular cementum which does not incorporate cells in its calcified matrix.
   
2. Cellular cementum which contains cementocytes in its calcified matrix. Layers of cellular and acellular cementum may alternate in any pattern.

   Afibrillar cementum: does not contain collagen fibers. Formed when the undifferentiated mesenchymal cells of the dental sac come in contact with the enamel surface.

   1-Acellular cementum:
   

   It is the first formed cementum layer. It is a thin layer predominates in the coronal half of the root. It may cover the root from the cemento-enamel junction to the apex, but it is often missing on the apical third of the root. Here cementum may by entirely cellular.

   

   Acellular cementum consists of calcified ground substance with embedded collagen fibers and lined by cementoid tissue with the cementoblasts on the surface.The bulk of its collagen fibers are formed of fully mineralized Sharpey’s fibers perpendicularly oriented, and few mineralized fibers constitute the intrinsic group run between them.In ground sections, Sharpey’s fibers are disintegrated and their spaces are filled with air, so they appear as dark lines.

   Cementoblasts :
   

   • Cementoblasts are derived from the undifferentiated mesenchymal cells of the tooth follicle (dental sac).

• 
• They are found lining the root surface interposed between bundles of PDL fibers.
• They synthesize collagen fibers (intrinsic) of cementum and its ground substance. It is also suggested that these cells may play a role in remodeling the ligament and cementum in certain cases.
• • Histologically active cementoblasts are round plump cells with basophilic cytoplasm indicative of an extensive RER. Under EM they show all the cytoplasmic organelles associated with a protein synthesizing and secreting cells. Resting cementoblasts are rounded smaller cells possess little cytoplasm and fewer organelles.

Cementoid:

Growth of cementum is a rhythmic process, as a new layer of cementoid is formed, the old ones become calcified. A thin layer of cementoid tissue is usually observed on the cemental surface.

The cementoid tissue is formed of the uncalcified cementum matrix which consists of:

• The collagen fibers: extrinsic from PDL, and intrinsic synthesized by the cementoblasts.
  
• The ground substance (proteoglycans, glycoproteins and phoshoproteins).
  

• The cementoblasts that lie on the outer surface.

2) Cellular cementum :

It is more frequent on the apical half of the root, and gradually increases in thickness as the apex is approached. It is frequently found on the surface of the acellular cementum. It is less in anterior teeth, but in multirooted teeth, it forms thick layer.

The collagen fibers of the cellular cementum constitute approximately equal proportions of the intrinsic and extrinsic types of fibers.

The characteristic feature of the cellular cementum is the presence of cementocytes occupying lacunae embedded in its mineralized matrix such as osteocytes in the bone.

Cementocytes are entrapped cementoblasts. They are oval spider like with sparse cytoplasm and possess numerous processes radiating from the cell body and occupying canaliculi.

These processes may branch and occasionally anastomose with those of the neighbouring cells. Most of the processes are directed toward the PDL surface of cementum which is the site of diffusion for essential nutrients since cementum is avascular.

In the deeper layers of cementum, the cementocytes become progressively further away from the nutritive source. They show few cytoplasmic organelles with dilated endoplasmic reticulum and are in stages of degeneration.

At the depth of 60 um or more, these cells show definite signs of degeneration such as cytoplasmic clumping and vesiculation.

The most deepest layer of the cellular cement may contain empty lacunae.

The lacunae are best seen in ground sections as black spaces with radiating canaliculi.

The Intermediate Cementum :

Recent investigations have confirmed the presence of an intermediate layer of cement on the surface of the roots, Thus it is situated between the granular layer of Tomes and the cementum.

This thin layer appears nearly identical to aprismatic enamel, that product of the ameloblasts which is 10 urn thick and covers the mantle dentin in the crown of the tooth.

It is best described as an amorphous layer of noncollagenous material containing no odontoblastic processes or cementocytes.

However some investigators believed that this layer represent areas where cells of Hertwig’s epithelial sheath become trapped in a rapidly deposited dentin or cementum maxtrix.

It has been suggested that it is formed by cells of the epithelial root sheath before its fragmentation.This layer mineralizes to an extent greater than that of either the adjacent dentin or cement. Its function is probably,

• To “cement” cementum to dentin.
  
  • To furnish an attachment of Sharpey’s fibers to the root surface.
    
• To seal the surface of the sensitive root dentin.
  

This layer is predominately seen in the apical two thirds of posterior teeth and only rarely observed in incisors or deciduous teeth.

Afibrillar cementum:

It is randomly distributed and consists of a thin acellular layer of a well mineralized ground substance devoid of collagen fibrils. It is difficult to identify by the light microscope.

It may cover cervical enamel or intervenes between fibriller cementum and dentin. It may be derived from epithelial origin.

Incremental Lines of Salter :

Both cellular and acellular cementum are formed in successive layers. The intervals between this successive deposition are represented by incremental lines of Salter. It is highly mineralized with less collagen fibers and more ground substance than other portions of the cementum. Incremental lines can be best seen in decalcified specimens prepared for light microscope. They are difficult to identify under EM.

Cementodentinal junction

This junction is relatively smooth in the permanent teeth, however it is sometimes scalloped in deciduous teeth.

It is clearly seen in decalcified stained sections by light microscope since cementum stains more intensely than dentine.

However, when viewed under EM, this junction is not well distinct due to the interwining of the collagen fibers of cementum and dentin. In decalcified preparations, cementum is more electron dense than dentine and some of its collagen fibrils are arranged in relatively distinct bundles, while those of dentin are arranged somewhat haphazardly.

Cementoenamel junction

The relation between cementum and enamel at the cervical region of the teeth is variable.

• In about 30% of the teeth the cementum meets the cervical end of enamel in a sharp line.
  
• In about 10% of the teeth, the enamel and cementum do not meet. Instead a zone of the root devoid of cementum (exposed dentin) is found. This may occur when part of the epithelial root sheeth in the cervical portion of the root is delayed in its separation from dentin preventing dental sac from coming in contact with dentin
  
• In about 60% of the teeth, cementum overlaps the cervical end of the enamel for a short distance. This occurs during development when enamel epithelium degenerates over the enamel at its cervical end permitting the connective tissue to come in contact with enamel and form cementum.
  

It is evident that the cementum formed is devoid of collagen fibrils and so termed afibriller cementum.

If such afibrillar cementum remains in contact with connective tissue cells for enough time, fibrillar cementum may subsequently be deposited on its surface.

Apically, the root sheath may remain attached to the dentin surface at a localized area and form enamel drop or pearls.